The invention concerns a process for the corrosion-resistant coating of metal substrates by means of plasma polymerization. The process is especially suitable for the corrosion-resistant coating of aluminum and aluminum alloys.
Ever since research began addressing the production of plasma-applied polymer layers by means of polymerization processes which produce the energy needed for polymerization by adding gaseous monomers to a gas discharge process, there has been no lack of attempts to deposit these layers in such a manner that they are able to protect the coated surface from different types of attack. This function is by no means trivial, since plasma-applied polymer layers are decidedly thin layers, measuring in the nanometer range up to a few micrometers. Not only were scratch-resistant layers developed, e.g. for optical functional elements made of plastic (WO-A-8504601), but there were also attempts, with moderate success, to use this type of layer to protect metallic materials. Even types of attack that must be considered less than severely corrosive were withstood by these layers for only very short periods of time.
In all the experimentation that is known at this time involving aluminum materials, oxide layers are used to promote adhesion in oxidizing plasma arrangements, and this is analogous to conventional lacquering processes, however, it is also analogous to the preparation of surfaces before gluing, where an oxide layer, which has usually been produced by anodic oxidation, is used. Activation of the boundary layer, which is desirable for good adhesion, is achieved, if at all, by intercalating substances of a foreign nature. Bonding is frequently carried out solely by means of adhesive forces. Experience has shown that such coating or gluing systems exhibit only moderate imperviousness to infiltration, because water vapor arising by diffusion or permeation processes weakens the bond between the material and the coating.
Plasma polymerization, on the other hand, is a process that is capable of coating solid objects by the action of a plasma on an organic molecule in the gas phase, whereby the coatings created in this manner are primarily organic in character and have excellent properties. Plasma polymerization belongs to the category of low-pressure plasma processes, and is used increasingly for industrial purposes. The great interest in this technology derives from the advantages of a rapid, contact-free, dry chemical coating process, which furthermore puts little stress on the work piece.
Plasma-applied polymer layers deposited by low-temperature plasmas, hereinafter referred to as plasma polymers, are distinguished by the following characteristics:
Plasma polymers are often three-dimensionally highly cross-linked and insoluble and swell only slightly or not at all, and are potentially good barriers to diffusion.
Compared to conventionally manufactured polymers, their high degree of cross-linking makes them unusually stable thermally, mechanically, and chemically.
The layers adhere well to most substrate materials, and have a high density and are free of micropores.
The layers are usually amorphous in structure, with a smooth surface that conforms to the shape of the substrate.
The layers are very thin, and the thickness of the layer amounts to only a few 100 nm down to 10 nm.
The process temperatures are low, i.e. room temperature up to approximately 100xc2x0 C., especially up to approximately 60xc2x0 C.
On the other hand, no processes are yet known with which metal substrates, especially substrates comprising aluminum materials, can be made corrosion-resistant by coating with a plasma polymer.
Ribbed pipes made from the material AlMgSi0.5 are frequently used in condensing boilers. When used under extreme conditions and in areas approaching the limits for allowable gas composition, such ribbed pipes do not always exhibit sufficient corrosion resistance.
The formation of corrosion products results in malfunctions on the gas side in the vicinity of the pipe ribs and, in advanced stages, a reduction of the heat exchanger surface on the combustion gas side, as well.
Conventional means for protecting against corrosion, which are state of the art, cannot be adopted for several reasons. Processes such as phosphatizing or chromizing bring about the continuous emission of heavy metal ions into the environment and must be excluded due to the likelihood of more restrictive anticipated legislation on waste water disposal.
Lacquer systems do not constitute a viable alternative, either. Lacquers used as a means for protecting surfaces compromise thermal conductivity, which in the present case can be tolerated only within very narrowly defined limits. Furthermore, in conventional lacquer coatings, the diffusion of water vapor leads to infiltration of the protective layer. Subsequent condensation on the metal surface causes the layer to separate in such systems, thereby accelerating the process of corrosion, as is known for localized types of corrosion.
Coating such ribbed pipes for heat exchangers with a plasma polymer would, in and of itself, be desirable. However, in experimental trials, in this connection, corrosion-resistant coatings were not achieved. As a rule, the plasma polymers were found not to adhere firmly enough to the metal surface, and infiltration of the coating was found to occur more or less rapidly, with the result that it soon showed signs of separating.
A process for the surface coating of silver objects is known from [the German patent application] DE-A-42 16 999, in which the surface is first treated with a stripping plasma, and the surface is then coated with a plasma polymer, whereby an initial coupling layer, a surface layer to prevent permeation on top of that, and finally a sealant layer are produced. Ethylene and vinyltrimethylsilane are especially used for the coupling layer, ethylene for the layer preventing permeation, and, for the sealant, hexamethyldisiloxane in conjunction with oxygen as a plasma forming monomer, whereby a continuous transition occurs between the plasma forming monomers. The coatings are largely scratch resistant, and they provide good protection against tarnish; in certain formulations, however, they can be susceptible to removal by cleansers. A coating on aluminum substrates fails to provide corrosion-resistant layers.
On the whole, it would be desirable to have a process available with which a long-lasting and corrosion-resistant plasma polymer coating can be applied to metal materials, especially aluminum materials.
This goal is achieved by a process of the type mentioned at the outset, where the substrate undergoes a pre-treatment step of smoothing by mechanical, chemical, and/or electrochemical means, after which, at a temperature of less than 200xc2x0 C. and a pressure of 10xe2x88x925 to 100 mbar, it is exposed to a plasma, whereby, in an initial step, the surface is activated in a reducing plasma and, in a second step, the plasma polymer is deposited from a plasma that optionally contains at least one hydrocarbon or organosilicon compound, which can be vaporized under the conditions of the plasma, containing oxygen, nitrogen, or sulfur, and which may contain fluorine atoms.
A surprising finding was that the problem of insufficient adhesion of the coating to the metal surface is solved by the combination of a smoothing pre-treatment of the metal substrate which is to be coated with a plasma treatment. The plasma treatment, in turn, consists of 2 steps, namely, first treatment of the surface by a reducing plasma which acts as a surface stripper, and a second step, in which the actual coating is applied directly to the metal layer that has been pre-treated by the plasma.
The pre-treatment, especially the smoothing of the surface of the metal substrate, can be carried out by mechanical, chemical, or electrochemical means. Especially preferred are combinations comprising both mechanical and chemical smoothing. In any event, electrochemical smoothing can be undertaken after mechanical and/or chemical smoothing, if the particular metal substrate in question will permit this. In the case of ribbed pipes, for example, the electropolishing method is not a suitable surface treatment for physical/technical reasons. Here one has to rely on chemical methods, such as acid or alkaline pickling. According to [the German patent document] DE-C-40 39 479, it is also possible to use a combination of pickling and mechanical disturbance of the surface by wiping, brushing, abrasive blasting processes, etc., whereby the work piece, in particular, is impacted by a jet of liquid containing both the pickling solution and particles with an abrasive action.
The pickling methods used in smoothing the surface are chemical processes in which aggressive chemicals are used to remove, primarily, layers of oxide, rust, and scale from the particular metal surface in question. Pickling solutions are usually acids that attack both the covering layers and the metal itself. Pickling is not a single process. Instead, different chemical and physical processes occur simultaneously and in succession. These processes are often electrochemical in nature, and involve the formation of local elements [sic] between the metal oxides and the metal surface.
Electropolishing is a method used for obtaining shiny metal surfaces, and it electrolytically removes raised bumps and ridges.
Chemical bright pickling is highly developed as a process for smoothing off surface roughness, especially for aluminum. It is fundamentally more important than electropolishing. There are many chemical bright picklers for aluminum.
Most chemical solutions for producing gloss are formulated on the basis of phosphoric acid. The addition of nitric acid produces reflecting surfaces as well as improving the surface quality. The addition of sulfuric acid makes the metal dissolve more quickly and results in the improved evening out of irregularities. There are other additives that can further speed up the rate at which the metal is stripped and extend the length of time the pickling bath will remain operational.
The effects of pickling, including bright pickling, can be made less variable and faster when they are used in conjunction with mechanical methods of surface treatment. According to the invention, such a combination of mechanical and chemical methods of surface treatment, as those described in [the German patent document] DE-C-40 39 479, is especially utilized for smoothing.
Due to the amphoteric properties of aluminum and its alloys, it is also possible to use alkaline solutions to clean and pickle them.
Generally speaking, the treatment to smooth the surface results in an average mean roughness of less than 350 nm, preferably less than 250 nm. By using electropolishing methods, and especially electropolishing after a mechanical/chemical smoothing process, an average mean roughness of less than 100 nm can be obtained.
However, surfaces that have been smoothed by such methods are still not optimally suited for the application of a plasma polymer. If a plasma polymer is applied after mechanical/chemical and/or electrochemical smoothing, it will not yet hold up for the desired length of time under corrosive conditions. In order to achieve that goal, one more surface treatment is needed, using a reducing plasma, especially a hydrogen plasma. Said plasma treatment takes place at temperatures of xe2x89xa6200xc2x0 C. and pressures of xe2x89xa6100 mbar, especially xe2x89xa6100xc2x0 C. and xe2x89xa610 mbar. Other gases can be added to the hydrogen as the plasma carrier, e.g. hydrocarbons, and olefins in particular, as described below, as well as oxygen, nitrogen, or argon, whereby the reducing character must always be maintained.
This plasma treatment produces an activated surface. The reducing conditions presumably cause a decrease in the aluminum oxide layer and/or aluminum hydroxides which are near the surface on the metal surface, so that points of attachment arise where reactive bonding of a plasma polymer, subsequently applied, can arise directly on the metal. A further side effect is that the surface undergoes additional smoothing by the plasma treatment.
A plasma polymer is precipitated onto the plasma-treated surface, preferably under further reducing conditions at first. The main component of this plasma polymer is a hydrocarbon and/or an organosilicon compound, which can contain oxygen, nitrogen, or sulfur atoms, whereby said hydrocarbon or organosilicon compound has a boiling point such that it can be vaporized under the temperature and pressure conditions prevailing in the plasma coating chamber. Substances that would qualify for this purpose are primarily alkanes, alkenes, aromatic hydrocarbons, silanes, siloxanes, silazanes, and silathianes, preferably siloxanes. The utilization of hexamethyldisiloxane and hexamethylcyclotrisiloxane is especially preferred. Other compounds are hexamethyldisilazane and hexamethylcyclotrisilazane, as well as hexamethyldisilathiane. It is also possible to use higher homologs of these compounds and mixtures of such compounds, or their partially or completely fluorinated derivatives.
Hydrocarbons, especially olefins, e.g. ethylene, propene, and cyclohexene, can serve as co-monomers for the formation of plasma polymers from organosilicon monomers. Silanes, especially organosilicon compounds containing vinyl groups, can also be used as co-monomers, e.g. vinyltrimethylsilazane. These unsaturated monomers can be added in fixed or variable proportions to the organosilicon compound containing O, N, or S atoms, with one possibility being to add them in gradual increments. For example, when forming the plasma polymer step by step, it is possible first to form a transition layer on the metal surface, consisting solely or predominantly of the organosilicon compound, and subsequently to add the hydrocarbon. Another possibility is to proceed in the reverse order. Thus, the properties of the plasma polymer coating can be modified so as to provide optimal adhesion to the metal substrate and/or optimal resistance to corrosive substances. Such a manner of forming the layer in stages is known, for example, from [the German patent application] DE-A-42 16 999.
During plasma polymerization, other gases in addition to these monomers can also be added, e.g. oxygen, nitrogen, or argon, in order to modify the properties of the plasma and the plasma polymer.
Generally speaking, plasma polymerization takes place at a temperature of xe2x89xa6200xc2x0 C., preferably xe2x89xa6100xc2x0 C., and especially about 60xc2x0 C. The pressure in the plasma coating chamber is usually xe2x89xa610 mbar.
The layer formed on the metal substrate by plasma polymerization expediently has a thickness of 100 nm to 10 xcexcm. But it is not a problem to create layers with a thickness of less than 100 nm for special purposes.
Unlike other coatings, including plasma polymer coatings that are applied by using other methods, the surface is smoothed according to the invention by a pickling process that evens it out, and whose effectiveness is enhanced and rendered less variable by a superimposed light mechanical component. As a result, mechanical cramping of the polymer coating on the metal substrate caused by the relatively high roughness of the substrate occurs less often, and instead there is bonding of a rather more chemical nature to free valences of the exposed metal surface that has been stripped by caustic agents. Generally speaking, a bright, almost mirror-like finish that appeals to the eye is obtained on non-structured metal surfaces. The result, in particular, is that, with its thickness being what it is, the coating is no longer xe2x80x9cswallowed upxe2x80x9d by the surface structures of a rough metal surface, and instead a uniform, smooth layer is formed.
Corrosion resistance several times better than that obtained for industrial surfaces is achieved according to the invention.
A further increase in long-term corrosion resistance is achieved by incorporating a corrosion inhibitor which can be vaporized in a vacuum, preferably into the bottom-most layer of the plasma polymer coating. In a departure from previously available findings, it is not essential for such a corrosion inhibitor to be applied directly to the surface of the substrate, i.e. to be located directly at the level where adhesion takes place, thereby weakening it. Instead, it is effective at a distance, especially in connection with the use of conductive polymers. Suitable polymers of this type are, for example, polyanilines which have low vapor pressure [sic] in a vacuum, or which can be introduced into the plasma polymer in the most finely distributed form, in quantities of 0.1 to 1% by weight.
In addition to being used with aluminum materials, the technology described above can be applied to other metal materials as well, especially those with a tendency to form an oxide layer on their surface.
The process according to the invention can also be used for applying a plasma polymer primer to a metal substrate to which additional coatings are subsequently applied. This allows corrosion-resistant coatings of considerable thickness to be obtained for the most diverse purposes, and which have sufficient thickness to stand up to abrasive wear. Ormocers are especially well suited for this purpose. The structural composition of ormocer coatings is similar to highly cross-linked plasma polymer coatings, but they can be formed without the relatively slow vacuum coating process. Here, the typical thickness of the layer is on the order of 1 to 100 nm. Using them in combination, corrosion properties are obtained of a quality that is comparable to and as good as with plasma polymer coatings alone.
The process according to the invention is especially well suited for coating aluminum materials, whereby the resulting corrosion resistance renders the aluminum material especially suitable for being employed as a heat exchanger and for manufacturing ribbed pipes for heat exchangers in condensing boilers.